Method and instruments for percutaneous arthroscopic disc removal, bone biopsy and fixation of the vertebral

ABSTRACT

The insertion points for guide pins used in percutaneous vertebral fixation by pedicle screw are determined by establishing an approach path on a CT image, measuring on the CT image the distance from the intersection of the approach path and the skin of the patient&#39;s back to the patient&#39;s midline, and marking off the corresponding distance while visualizing the patient by anteroposterior fluoroscopy. An longitudinally expansible pedicle screw comprises a shaft with bone-engaging threads on a distal portion, machine screw threads on a proximal portion, and a tubular extension having wrench-engageable holes on a proximal face, and internal threads engaging the machine screw threads on the proximal portion of the shaft. An oval cannula accommodates an arthroscope and forceps simultaneously for intervertebral disc surgery.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 09/370,318, filed Aug. 9,1999, now U.S. Pat. No. 6,175,758, which is a continuation-in-part ofapplication Ser. No. 08/893,286, filed Jul. 15, 1997, now U.S. Pat. No.5,964,761.This application is a reissue of U.S. application Ser. No.09/711,614, filed Nov. 13, 2000, now U.S. Pat. No. 6,596,008, which is adivision of application Ser. No. 09/370,318, filed Aug. 9, 1999, nowU.S. Pat. No. 6,175,758, which is a continuation-in-part of applicationSer. No. 08/893,286, filed Jul. 15, 1997, now U.S. Pat. No. 5,964,761.

SUMMARY OF THE INVENTION

This invention relates generally to surgery, and more particularly tomethods and instrumentation having their principal utility in spinalsurgery.

My U.S. Pat. Nos. 5,242,443, dated Sep. 7, 1993 and 5,480,440, datedJan. 2, 1996, the disclosures of which are here incorporated byreference, describe a surgical technique for percutaneous fixation oftwo or more adjacent vertebrae by means of screws insertedpercutaneously into the pedicles of the vertebrae and secured togetherby links located just under the skin of the patient's back. Thetechnique avoids the many difficulties encountered in conventionalinternal and external vertebral fixation.

In vertebral fixation according to the methods described in my priorpatents, a cannulated tubular guide is maneuvered into alignment withthe pedicle. A pin is introduced through the guide and tapped with amallet so that it enters the cortical bone at the junction of the baseof the transverse process and the proximal articular process. The guideis then removed and a cannulated obturator is placed over the pin. Anaccess cannula is then placed over the obturator and advanced to thepedicle. The obturator is then removed from the access cannula and acannulated drill is advanced over the pin and operated to form anentrance into the medullary canal of the pedicle. A probe is thenadvanced into the medullary canal to create a bore into the vertebralbody. The bore may then be tapped to form threads engageable by apedicle screw, or alternatively a self-tapping pedicle screw can beinserted. With pedicle screws threaded into pedicles of adjacentvertebrae, adapters of the appropriate length are selected and securedto the proximal ends of the screws. The screws, with the adaptersattached to them, are connected by links located just underneath thepatient's skin. The procedure is much less invasive than conventionalinternal fixation, minimizes damage to muscle tissue and ligaments,reduces recovery and rehabilitation time, and simplifies removal of thefixation appliances. The procedure also reduces the infection risks, andavoids the physical limitations, imposed on the patient by externalfixation.

The placement of a plate or rod under the skin, following commonlypracticed open spinal fusion and pedicular screw insertion, facilitatessubsequent retrieval of the hardware when the fusion and stabilizationare accomplished. This eliminates the need for a second operation foraccess to, and retrieval of, the deeply positioned hardware.

The internal diameter of the medullary canal of the lumbar pedicles istypically only about 7 to 8 mm. The small size of the medullary canalmandates precise positioning of screws in the pedicle. To achieve properalignment of the cannulated tubular guide with the axis of the pedicle,the guide is visualized fluoroscopically as it is being inserted. Whenproperly aligned, the guide appears as an opaque circle in the center ofthe pedicle. A similar fluoroscopic method is used for alignment of thescrew with the pedicle, the screw appearing as a dot in the center ofthe pedicle when properly aligned. This method is referred to as the“bulls-eye” method.

Conventional bulls-eye alignment has proven to be less than entirelyreliable for proper positioning of percutaneously inserted guides andscrews for several reasons. First, the operation is carried out using aradiolucent table and a C-arm fluoroscope. To allow better visualizationof the pedicles the table is tilted away from the C-arm, oralternatively, the C-arm is tilted relative to the table. The angle oftilt of the C-arm fluoroscope relative to the operating table affectsthe angle of the guide or screw. Therefore, accurate measurement of thenecessary tilt of the C-arm to accommodate the desired angle ofinsertion of the guide or screw is necessary. A second deficiency of theconventional bulls-eye alignment method arises because the skin entrypoint plays a significant role in the proper positioning of the guide atthe center of the pedicle and the insertion of the probe into thevertebral body. A third deficiency of the bullseye method is that thedistance between the guide and the x-ray tube can have an effect on theposition of the guide relative to the pedicle. If the x-ray tube is tooclose to the patient and the angle of the tilt of the C-arm has not beenpredetermined and measured, the peripheral x-ray beams may present adistorted view of the position of the guide. A fourth deficiency of thebulls-eye method is that, when the C-arm is tilted by 20° to 30°, clearvisualization of the boundaries of the pedicles is difficult.

Similar difficulties are encountered in positioning a biopsy cannula fortaking a vertebral biopsy. An important object of this invention,therefore, is to provide a more accurate and reliable method forestablishing an insertion point for a percutaneously inserted instrumentfor spinal surgery.

A method for determining the point for insertion of an instrument in apercutaneous spinal procedure in accordance with the invention,comprises the following steps. The patient is scanned by computedtomography, and an image is produced of an axial plane through thepatient, i.e. a plane perpendicular to the long axis of the patient'sbody. On that image, a desired path for insertion of a guide pin isdetermined, and the lateral distance from the patient's midline to thepoint at which said path intersects the skin of the patient's back isdetermined. By viewing the patient's spine radiographically in theanterior-posterior direction, the skin of the patient's back is markeddirectly over the midline, and marking the skin of the patient's back isalso marked with a line extending transverse to the patient's midline ina plane corresponding to the transverse plane in which the computedtomography scan was taken. Thereafter, an insertion point is establishedon the patient's back, on the transverse line, at a distance equal tothe lateral distance measured on the image.

Another important object of this invention, therefore, is to provide anmore accurate and reliable method for placement of a guide in the centerof a pedicle in preparation for the insertion of a pedicle screw orbiopsy cannula.

In accordance with the invention, the insertion points and insertionangles for the guide pins are established by a technique using acombination of computed tomography and conventional radiographicvisualization. CT scans of the patient are taken in axial planes throughthe pedicles of two or more vertebrae to be fixated. Using the CTimages, a desired path for insertion of a guide pin into each of thesevertebrae is established. The angle of the path relative to the medianplane is measured on the CT image for each of the vertebrae. Also, foreach of these vertebrae, a measurement is made, on the CT image thereof,of the lateral distance from the patient's midline to the point at whichthe insertion path intersects the skin of the patient's back. By viewingthe patient's spine radiographically in the anterior-posteriordirection, a marking is made on the skin of the patient's back directlyover the midline, and a transverse line is drawn for each of thevertebrae to be fixated over the centers of a pedicle thereof. Then, foreach of the vertebrae to be fixated, an insertion point is establishedon the patient's back, on the corresponding transverse line, at adistance equal to the lateral distance measured on the CT image thereof.Thereafter a guide pin is inserted through the patient's back, and intoa pedicle of each of the vertebrae to be fixated, through the insertionpoint established therefor and at the angle measured therefor.

Another problem with the method of percutaneous fixation as described inmy prior patents is that the adapters that are attached to the proximalends of the pedicle screws need to be available in a variety of sizes sothat, during surgery, pedicle screws of the appropriate lengths can beselected to position the connecting link at the proper position justunderneath the patient's skin. Often the available adapters are eithertoo long or to short to allow proper positioning of the connectinglinks. Moreover, during surgery, the attachment of the adapter to thescrew is time consuming and at times difficult. Removal and reinsertionof adapters in an effort to find one of the proper length can alsoresult in loosening and displacement of the pedicle screw. Still furtherobjects of the invention, therefore, are to simplify the procedure forvertebral fixation by eliminating the need for various different sizesof adapters, to allow easier and more precise positioning of theconnecting links, and to reduce the likelihood of loosening ordisplacement of pedicle screws.

In accordance with the invention, the step of inserting a guide pinthrough the patient's back is followed by the step of introducing alengthwise expansible pedicle screw, having proximal and distal ends,into the pedicle of each of the two or more vertebrae to be fixated.When each screw is properly introduced, its distal end is located insidethe vertebra and its proximal end is located underneath the fascia ofthe patient's back. Each screw is then expanded lengthwise until itsproximal end is located adjacent to the fascia, but underneath the skin.The proximal ends of the pedicle screws are then rigidly connectedtogether by connecting means located between the fascia and the skin ofthe patient's back.

A preferred expansible pedicle screw in accordance with the inventioncomprises a shaft having distal and proximal portions. The distalportion has threads adapted to be threaded into the medullary canal of avertebral pedicle, and the proximal portion has threads adapted toengage internal threads of a tubular extension. At its proximal end, thetubular extension has means, engageable by a wrench, for rotating theextension relative to the shaft in a direction to increase the distancebetween the proximal end of the tubular extension and the distal end ofthe shaft. Means are also provided for connecting a rigid link to saidproximal end of the extension, whereby the extension of the expansiblepedicle screw can be connected to another pedicle screw. Preferably, theshaft has a lengthwise internal passage for receiving a guide pin, andthe tubular extension has a threaded female recess communicating withthe passage for receiving a threaded adapter. A plurality of holes ispreferably provided in an end face of the tubular extension forreceiving projections of a wrench.

In the assembled vertebral fixation device, which comprises at least twoexpansible pedicle screws, the threads of the extensions are engagedpartway with the threads of the shafts so that engagement of the linkwith the extensions prevents the extensions from rotating relative theshafts in directions such as to shorten the lengths of the expansiblepedicle screws. The device therefore eliminates the need to keepmultiple adapters of different sizes on hand during surgery, avoids theproblem of selecting an adapter having the proper length, and allowsprecise positioning of the connecting link.

In various percutaneous spinal procedures, including decortication ofvertebral plates, insertion of bone grafts, removal of herniated discmaterial and resection of nuclear tissue, it is often desirable toutilize two cannulae at the same time, one for insertion of manipulativeinstrumentation, e.g. forceps, and the other for endoscopic observation.My U.S. Pat. No. 5,395,317, dated Mar. 7, 1995, describes suchpercutaneous spinal procedures, and specifically relates to a techniquein which a guide attached to a first cannula is used to maintain asecond cannula in parallel relation to the first cannula.

Still further objects of this invention include the simplification ofsuch percutaneous spinal procedures by obviating the second cannula, andthe simplification of the procedure for gaining access to theintervertebral disc.

In accordance with the invention, a specially designed cannula assemblyis used. The cannula assembly comprises a cannula having a wall ofuniform thickness defining a cylindrical inner passage having an ovalcross section, and an oval obturator located within the oval,cylindrical inner passage. The obturator has a cylindrical outer wallwith an oval cross-section and closely fits the cylindrical innerpassage of the cannula. The obturator preferably has an internal passagefor receiving a guide, and has a blunt end projecting from the distalend opening of the cannula.

The cannula assembly is introduced percutaneously through the back of apatient, and the obturator is removed. After removal of the obturator,by virtue of the oval cross-section of the cannula passage, two or moreinstruments, for example an arthroscope and a forceps, can extendthrough the cannula simultaneously.

The introduction of the oval cannula is preferably carried out byinserting an elongated, hollow sleeve percutaneously toward anintervertebral disc of a patient posterolaterally, while the sleeve hasan obturator extending through it; removing the obturator from thesleeve, while leaving the sleeve in place; passing over the sleeve anassembly comprising a cannula having an lumen with an oval cross-sectionand an obturator having an oval cross-section conforming to the lumen ofthe cannula, the oval obturator having a longitudinal passage forreceiving the sleeve; removing the oval obturator from the cannula,while leaving the cannula in place.

A trephine may be inserted through the cannula toward the intervertebraldisc, to form a fenestration in the annulus fibrosis of the disc, andthe cannula can then pass through the fenestration. Preferably, thelongitudinal passage in the oval obturator is large enough to receivethe trephine. This allows the trephine to be operated while the ovalobturator in still in place in the cannula, for more accurate guidanceof the trephine.

The annular fenestration may be performed, under direct arthroscopicvisualization and magnification, by a long and thin handle knife whichis introduced into the lumen of the oval cannula.

Other objects, details and advantages of the invention will be apparentfrom the following detailed description when read in conjunction withthe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view depicting a CT scan through the pedicles of alumbar vertebra, used for lateral location of an entry point and toestablish an angle of approach through a pedicle;

FIG. 2 is a schematic view depicting an anteroposterior fluoroscopicview of two adjacent vertebrae and their intervertebral disc, withradiopaque needles positioned on the patient's back for location of theentry points along the lengthwise direction of the spine;

FIG. 3 is a schematic view of a patient's back, showing a verticalmarking made over the spinal processes, two horizontal markings over thepedicles of adjacent vertebrae, and entry points on the horizontalmarkings;

FIG. 4 is a schematic anteroposterior view of a vertebra showing theposition of a guide pin in relation to a pedicle as the guide pin isabout to enter the pedicle;

FIG. 5 is a schematic lateral view of the guide pin in relation to thepedicle as in FIG. 4;

FIG. 6 is an axial section of an expansible pedicle screw in accordancewith the invention;

FIG. 7 is a top plan view of the expansible pedicle screw of FIG. 6;

FIG. 8 is an exploded view showing two expansible pedicle screws, arigid connecting link, and adapters and nuts for securing the connectinglink to the pedicle screws;

FIG. 9 is an elevational view of a sleeve for an obturator used inconjunction with an oval cannula in accordance with the invention;

FIG. 10 is an elevational view of the obturator;

FIG. 11 is a longitudinal section through an assembly consisting of thesleeve and obturator of FIGS. 9 and 10;

FIG. 12 is a longitudinal section through an oval obturator inaccordance with the invention;

FIG. 13 is a bottom plan view of the oval obturator of FIG. 12;

FIG. 14 is an elevational view of an oval cannula in accordance with theinvention;

FIG. 15 is a bottom plan view of the oval cannula of FIG. 14;

FIG. 16 is a longitudinal section through an assembly of the ovalcannula and obturator of FIGS. 12-15; and

FIG. 17 is an elevational view of a trephine used with the ovalobturator of FIG. 16.

DETAILED DESCRIPTION

The CT scan depicted in FIG. 1 shows a vertebral body 20 and acorresponding spinal process 22 in relation to the skin of the patient'sback 24. A line 26 is drawn on the CT scan through the spinal processand through the center of the vertebral body. An oblique line 28 isdrawn through the center of pedicle 30 to represent the desired path ofentry of a guide pin. This line will normally, although not necessarily,intersect line 26 approximately at the point at which line 26 passesthrough the anterior surface of the vertebral body. A measurement isthen taken of the distance D between line 26 and the point 32 at whichline 28 intersects the skin of the patient's back. The approach angle θbetween lines 26 and 28 is also measured by means of a protractor andnoted. The same procedure is carried out on a CT scan of each of the oneor more other vertebrae which are to be fixed to the vertebra shown inFIG. 1.

Either before or after the CT scans are taken, the patient is placedprone on a radiolucent frame and operating room table. The lumbarlordosis is flattened by flexion of the hips and the table, and care istaken to ensure that the patient is positioned symmetrically in relationto the frame and table. Then, a C-arm fluoroscope is positioned foranteroposterior radiographic visualization of the vertebral bodies atthe surgical site. For improved visualization of the pedicles, the C-armcan be tilted away from the pedicles, for example at an angle of 10degrees. However, the radiographic observation must be taken at thepatient's midline.

The radiographic view of the patient's spine, as seen through afluoroscope, is depicted in FIG. 2, where two adjacent vertebral bodies20 and 34 are shown, with an intervertebral disc 36 between them. Whileobserving the patient's spine through the fluoroscope, the surgeonplaces a first straight, narrow, radiopaque element 38, for example an18 gauge needle, over the spinal processes 22 and 40. When the element38 is properly positioned, the surgeon draws a vertical line in inkdirectly on the skin of the patient's back at a location correspondingto the position of the element 38. Similar radiopaque elements 42 and 44are positioned over the centers of pedicles 30 and 45 respectively andarranged perpendicular to the direction of element 38. Horizontal inklines are then drawn on the skin of the patient's back at locationscorresponding to the locations of elements 42 and 44.

As seen in FIG. 3, the ink lines on the patient's back 24 are shown at46, 48 and 50. A first entry point mark 52 is made on horizontal line 48at a distance D from the vertical line 46, the distance D having beenpreviously measured on the CT scan through vertebral body 20 and pedicle30. In a similar manner a second entry point mark 54 is made onhorizontal line 50 at a distance D′ from vertical line 46, D′ havingbeen measured on a CT scan of vertebral body 34.

Using a goniometer, a guide pin 56 (shown in FIGS. 4 and 5) is insertedthrough the skin at the entry point 52 (FIG. 3) and passed through thefascia until its tip reaches the pedicle 30 of vertebral body 20. Theposition of the tip of the guide pin, which should be just lateral toboundary of the pedicle 30 is verified by anteroposterior fluoroscopy asin FIG. 4, and appropriate adjustment can be made, if necessary.Likewise, the position of the pin in the middle of the pedicle andparallel to the vertebral plate is verified by lateral fluoroscopicobservation, as in FIG. 5, and again adjustments can be made ifnecessary.

The guide pin 56 is then hammered into the pedicle and into thevertebral body. This is done for the corresponding pedicles of each ofthe vertebrae to be fixated. A protective cannula may be placed over thepedicle for protection and retraction of the paravertebral muscles whilethe medullary canal of the pedicle us being tapped. A sound isintroduced to make certain that the cortex of the pedicle has not beenviolated.

After the medullary canal of the pedicle is prepared, a guide pin isinserted and a pedicle screw is passed over the guide pin and turned bya wrench until it passes the mid point of the anteroposterior diameterof the vertebra. The protective cannula may be left in place while thepedicle screw is being inserted.

The lengthwise expansible pedicle screw 58, which is shown in FIG. 6,comprises a shaft 60 having a distal portion 62 with threads adapted tobe threaded into the medullary canal of a vertebral pedicle. The shaft60 has a centrally located, multifaceted head 64 for engagement by awrench, for example a hex wrench, and a proximal portion 66 havingmachine screw threads 68. A tubular extension 70 has internal threads72, which are engaged with threads 68, so that rotation of the extensionrelative to the pedicle screw shaft 60, causes the extension to movelengthwise in one direction of the other, thereby increasing ordecreasing the effective length of the pedicle screw 58. The threads 68and 72 should fit each other with a close tolerance so that theextension and shaft form a rigid unit when they are locked againstrelative rotation.

The pedicle screw shaft 60 has an central passage 74 extendinglengthwise through it along its axis of elongation. This central passageenables the shaft to be placed over a guide pin. The upper end of thetubular extension has a threaded recess 76, with a bottom opening 78aligned with passage 74, enabling the guide pin to pass through upperend of the extension. As shown both in FIGS. 6 and 7, holes 80 and 82are provided in upper end face 84 of the extension 70 for receivingprojections of a special wrench for rotating the extension. The upperend of the extension 70 is also formed with wrench-engageable facets 85,which allow it to be held against rotation by a wrench as an adapter isattached to it. The facets 85 form a hex nut which is smaller than hexhead 64, thus allowing an elongated socket wrench to be inserted overthe extension 70 for engagement with hex head 64.

As shown in FIG. 8, the threaded recess 76 of the extension 70 isengageable with threads 86 of an adapter 88. The threads 86 extenddownward from a plate 90. The plate has notches 92 and 94 for engagementby a special wrench used to tighten it into secure engagement with theextension 70. These notches are at opposite ends of a raised, part 96 ofthe plate, the raised part having parallel edges 98 and 100. The raisedpart 96 fits into a slot 102 in a link plate 104, and the adapter issecured to the link plate by a nut 106, which engages threads 108 whichextend upwardly from the upper face of the adapter. The thickness ofraised part 96 of the adapter is slightly less than the thickness ofplate 104. Mating serrations 110 and 112 are provided on the adapter andon the link plate respectively.

FIG. 8 also shows an identical lengthwise expansible pedicle screw 58′,which is connectible to a second rectangular slot 102′ in link plate 104by means of an adapter 88′ and a nut 106′.

With the pedicle screws 58 and 58′ inserted into the correspondingpedicles of adjacent vertebrae, the guide pins are removed. Theextensions are then rotated counterclockwise by a special wrenchinserted into the holes 80 and 82 (see FIG. 7) until the outer ends ofthe extensions are at a level above the lumbar fascia. With theextensions projecting above the lumbar fascia, their faceted outer endscan be engaged and held against rotation by another wrench so thatadapters 88 and 88′ can be attached. The adapters are secured by aspecial wrench engaged with their notches 92, 94, 92′ and 94′. After theadapters are secured to the extension, further adjustment, if necessarycan be carried out by rotating the adapters and extensions together.Then, through an appropriate incision, the link plate 104 is insertedand secured to the adapters. The link plate is located just above thesurface of the lumbar fascia but below the skin.

The procedure for locating the insertion points and for determining theangle of approach for the guide pins provides for accurate placement ofthe pins and of the pedicle screws, eliminates the distortion inherentin previous methods, reduces the likelihood of damage to the vertebralstructure, and produces generally superior and consistent results. Thelocation procedure, using a combination of CT scanning andanteroposterior radiographic observation, is also usable for accuratedetermination of the insertion point and approach angle for a biopsycannula and for various other procedures in which a spinal instrument isinserted percutaneously.

The lengthwise expansible pedicle screw is easily inserted and adjusted,and has as its principal advantage the fact that it eliminates the needto keep adapters of various lengths on hand for possible use in surgery.

For percutaneous spinal procedures such as decortication of vertebralplates, insertion of bone grafts, removal of herniated disc material andresection of nuclear tissue, the instrumentation depicted in FIGS. 8-17can be used.

An obturator 114, shown in FIG. 10 is inserted into a guide sleeve 116,shown in FIG. 9, and the assembly of the obturator and sleeve, as shownin FIG. 11, is passed through the skin, subcutaneous tissue and musclelayer. The skin entry point can be determined by the size of the patientand a combination of CT scanning and anteroposterior and lateralradiographic observation in the same manner as in the case of theinsertion of pedicle screws. Typically, the entry point is about 10-12cm. from the midline. In an anteroposterior fluoroscopic projection, thetip of the obturator 114 is located either at the mid pedicle or lateralpedicle line. In a lateral fluoroscopic projection, the tip of theobturator and sleeve assembly should be touching a line drawn posteriorto the backs of the vertebrae adjacent to the intervertebral disc whichis being accessed.

An obturator 118, having a longitudinal passage 120 and a blunt end 122,as shown in FIGS. 12 and 13, is inserted into a cannula 124, as shown inFIGS. 14 and 15. The cannula has an oval cross-section, and the body ofthe obturator 118 also has an oval cross-section, conforming to theinterior wall of the cannula. The major axis of the lumen of the cannulais preferably in the range of 7 to 12 mm, and the minor axis ispreferably in the range of 3 to 5 mm. Thus, a typical cannula can have a3×7 mm lumen, a 5×12 lumen, or even a 3×12 or 5×7 lumen.

The obturator 114 is removed from the sleeve 116, and the assembly ofthe cannula and obturator, as shown in FIG. 16, is passed over the guidesleeve 116 and directed toward the annulus fibrosis. The obturator andguide sleeve are then removed, leaving the cannula 124 in place.

A zero degree arthroscope is inserted to inspect the annular surface tomake certain that neural structures are not in the path of the insertedcannula. After carrying out any necessary hemostasis, a trephine 126,shown in FIG. 17, is inserted through the passage in the obturator tosever the annular fibers, and the oval cannula is inserted into thefenestration formed by the trephine and engaged into the annular fibers.The trephine typically has an outer diameter of 4.5 mm.

The annular fenestration may also be performed, under directarthroscopic visualization and magnification, by a long and thin handleknife which is introduced into the lumen of the oval cannula.

The oval cannula also may be used for the insertion of expandable cageswhich are utilized for fusion of the two adjacent vertebrae or theintroduction of a disc prosthesis.

When the cannula is being used for the retrieval of herniated discfragments, an attempt should be made to place the inner end of theinserted cannula adjacent to the spinal canal. In order to accomplishthis, a zero degree arthroscope is inserted into the lumen of the ovalcannula, and the inner end of the cannula is tilted toward the dorsum ofthe patient. This maneuver permits the visualization of the dura or thenerve root. At this time, the cannula is tilted ventrally and heldagainst the annulus in preparation for annular fenestration and removalof herniated disc fragments.

When obtaining tissue from the vertebral bodies for biopsy anddefinitive diagnosis is desired, the steps which were described forinsertion of a guide pin into the pedicle are followed by introductionof a cannulated drill with an outer diameter not exceeding 7 mm over theguide pin. The drill is driven through the medullary canal of thepedicle into the vertebral body of the desired vertebra.

A cannula is then inserted into the pedicle following the removal of thecannulated drill. Straight and upbiting biopsy forceps is passed throughthe cannula for harvesting of abnormal tissue.

The oval shape of the lumen of the cannula permits simultaneousintroduction of an arthroscope and forceps for removal of herniated discmaterial and resection of nuclear tissue.

Where interbody fusion is to be attempted, another portal on theopposite side of the spine is necessary so that an auxiliary cannula canbe introduced. However, this auxiliary cannula can be of comparativelysmall diameter, e.g. 5 mm. Curettes and reamers are passed through theoval cannula while an arthroscope, e.g. a 30 or 70 degree arthroscope isintroduced into the intervertebral disc through the auxiliary cannulafrom the opposite portal. Decortication of the vertebral plates iscarried out under direct arthroscopic magnification and illumination.

Autogenous bone, harvested from the patient's ilium, is passed throughthe cannulae and packed between the vertebral plates of the proximal anddistal vertebrae. Afterwards, the instruments are withdrawn and thewound is closed.

The oval cannula has the advantage that it allows plural instruments,e.g. an arthroscope and a forceps to extend through it at the same time,thereby eliminating the need for a separate cannula on the same side ofthe spine as in U.S. Pat. No. 5,395,317. The oval cross-section is alsoadvantageous in that it accommodates decorticators having blades withcomparatively large lateral dimensions, e.g. 7 mm or 9 mm, for access tothe concave surfaces of the vertebral plates. The decorticator can berotated after the blades at its distal end pass beyond the opening atthe distal end of the oval cannula. The use of the oval obturator allowsthe oval cannula to be introduced readily with a minimum number ofsteps, and provides guidance for the trephine used to fenestrate theannulus fibrosis of the disc.

Various modifications can be made to the instruments and proceduresdescribed herein. For example, the sequence in which the CT scans andthe radiographic visualization take place can be reversed. Also, thestructure of the longitudinally expansible pedicle screws can bemodified, for example by providing for alternative shapes andconfigurations for the wrench-receiving elements 64 and 80 and 82.Various alternative forms of adapters and link plates or rods can alsobe used. Still other modifications may be made to the apparatus andmethod described above without departing from the scope of the inventionas defined in the following claims.

I claim:
 1. A cannula assembly for use in arthroscopic spinal surgerycomprising: a guide sleeve having a passage therethrough; a firstobturator located within the passage of the guide sleeve; a cannulahaving a wall of uniform thickness, the wall defining a cylindricalinner passage having an oval cross section,; and ana second obturatorlocated within said cylindrical inner passage of said cannula, theobturator having a cylindrical outer wall with an oval cross-section andclosely fittingconforming to the cylindrical inner passage of thecannula, the second obturator having an internal passage therethroughconfigured to receive the guide sleeve; wherein the inner passage of thecannula is sized and configured to allow a plurality of instruments toextend through the passage simultaneously.
 2. A cannula assemblyaccording to claim 1, further comprising a trephine, wherein theobturator has an internal passage of the second obturator is sized forreceiving a guide the trephine.
 3. A cannula according to claim 1,wherein the cannula has proximal and distal end openings and theobturator has a blunt end projecting from the distal end opening of thecannula.
 4. A method of spinal surgery comprising the steps ofintroducing percutaneously through the back of a patient a cannulaassembly comprising a cannula having a wall of uniform thickness, thewall defining a cylindrical inner passage having an oval cross section,and an obturator located within said cylindrical inner passage, theobturator having a cylindrical outer wall with an oval cross-section andclosely fitting the cylindrical inner passage of the cannula, removingthe obturator, and introducing at least two instruments through thecannula whereby said at least two instruments extend through the cannulasimultaneously.
 5. A method of spinal surgery according to claim 4, inwhich a fenestration is formed in the annulus fibrosis of a vertebraldisc of the patient, in which the cannula is inserted into thefenestration, in which said at least two instruments include anarthroscope and a forceps, and in which herniated disc material isremoved from the disc by means of the forceps.
 6. A method of spinalsurgery comprising the steps of: inserting an elongated, hollow sleevepercutaneously toward an intervertebral disc of a patientposterolaterally, while the sleeve has an obturator extending throughit; removing the obturator from the sleeve, while leaving the sleeve inplace; passing over the sleeve an assembly comprising a cannula havingan lumen with an oval cross-section and an obturator having an ovalcross-section conforming to the lumen of the cannula, the obturator withthe oval cross-section having a longitudinal passage for receiving thesleeve; removing the obturator having the oval cross-section from thecannula, while leaving the cannula in place.
 7. A method of spinalsurgery according to claim 6, in which a fenestration is formed in theannulus fibrosis of said disc, and the cannula is inserted into thefenestration.
 8. A method of spinal surgery according to claim 6, inwhich a trephine is passed through the cannula toward saidintervertebral disc, a fenestration is formed in the annulus fibrosis ofsaid disc by means of the trephine, and the cannula is inserted into thefenestration.
 9. A method of spinal surgery according to claim 6, inwhich, prior to the removal of the obturator having the ovalcross-section from the cannula, a trephine is passed through thelongitudinal passage in the obturator toward said intervertebral disc,and a fenestration is formed in the annulus fibrosis of said disc bymeans of the trephine, and after the removal of the obturator having theoval cross-section from the cannula, the cannula is inserted into thefenestration.
 10. A method of spinal surgery according to claim 6, inwhich, after removal of the obturator having the oval cross-section fromthe cannula, at least two instruments are introduced through the cannulawhereby said at least two instruments extend through the cannulasimultaneously.
 11. A method of spinal surgery according to claim 6, inwhich, after removal of the obturator having the oval cross-section fromthe cannula, an arthroscope and a forceps are introduced through thecannula whereby the arthroscope and forceps extend through the cannulasimultaneously, and herniated disc material is removed from the disc bymeans of the forceps.
 12. A method of spinal surgery according to claim6, in which a trephine is passed through the cannula toward saidintervertebral disc, a fenestration is formed in the annulus fibrosis ofsaid disc by means of the trephine, the cannula is inserted into thefenestration, and an arthroscope and a forceps are introduced throughthe cannula whereby the arthroscope and forceps extend through thecannula simultaneously, and herniated disc material is removed from thedisc by means of the forceps.
 13. A method of spinal surgery accordingto claim 6, in which, prior to the removal of the obturator having theoval cross-section from the cannula, a trephine is passed through thelongitudinal passage in the obturator toward said intervertebral disc, afenestration is formed in the annulus fibrosis of said disc by means ofthe trephine, and after the removal of the obturator having the ovalcross-section from the cannula, the cannula is inserted into thefenestration, and an arthroscope and a forceps are introduced throughthe cannula whereby the arthroscope and forceps extend through thecannula simultaneously, and herniated disc material is removed from thedisc by means of the forceps.
 14. A method for accessing a spinallocation of a patient, comprising: advancing a first tissue-displacingmember to the spinal location; advancing a guide member to the spinallocation over the first tissue-displacing member; inserting a secondtissue-displacing member over the guide member, the secondtissue-displacing member comprising an elongate body with a proximal endand a tapered distal end, the proximal end of the secondtissue-displacing member once inserted being positioned outside thepatient and the distal end of the second tissue-displacing member onceinserted being positioned adjacent the spinal location; the elongatebody having a substantially constant acircular outer circumference inlateral cross-section over a portion of the length of the elongatemember in the patient; wherein the second tissue-displacing memberdefines an interior passage that runs the length of the elongateportion, the interior passage exiting the interior of thetissue-displacing member at the tapered end; and advancing an elongateaccess device defining an interior passage to the spinal location overthe second tissue-displacing member, the elongate access devicedimensioned to provide surgical access to the spine.
 15. The method ofclaim 14, wherein the tissue-displacing member has a major axis and aminor axis in lateral cross-section, wherein the major axis is largerthan the minor axis.
 16. The method of claim 14, wherein thetissue-displacing member has an oval cross-section.
 17. The method ofclaim 14, wherein the guide member is a sleeve.
 18. The method of claim14, wherein the interior passage of the elongate access device isacircular.
 19. The method of claim 14, wherein the interior passage ofthe elongate access device has a major axis and a minor axis in lateralcross-section, wherein the major axis is larger than the minor axis. 20.The method of claim 14, wherein the interior passage of the elongateaccess device is oval.
 21. The method of claim 14, wherein the elongateaccess device is a cannula.
 22. The method of claim 14, furthercomprising removing the first and second tissue-displacing members fromthe elongate access device and performing treatment through the accessdevice.
 23. The method of claim 22, wherein performing treatmentcomprises inserting multiple instruments simultaneously through theaccess device to the spinal location.